We have built a high-DC-voltage GaAs photoemission gun and used it to measure the rms transverse emittance and rms momentum spread of the highly-charged, short-duration electron bunches it generates. We report these measurements as functions of microbunch charge for different beam radii, pulse lengths, and voltages/field gradients at the cathode, and compare them with the theoretical predictions of the computer program PARMELA.
The measurements were performed at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, using the photoemission gun, a drive laser, and a short diagnostic beamline. While the electron gun was designed to operate at 500 kV, field-emission-induced ceramic punch-throughs forced us to limit beam energy to 250 and 300 kV for this experiment. A modelocked, frequency-doubled, Nd:YLF laser served as the photo-cathode driver, producing beams with rms lengths between 20 and 25 ps. The beamline used a scanning slit, wire scanners, and a dipole magnet to measure the transverse emittance and momentum spread for beam conditions ranging from the emittance-dominated to the space-charge-dominated regimes.
As the microbunch charge was varied from 0.5 to I25 pC, the transverse emittance increased from 0.2 to 2.3 n mm mr. Using PARMELA estimates of the bunch lengths at the measurement plane, we calculated that the normalized peak brightness of these bunches varied from 10^9 to 10^10 A m-2 rad-2 over the charge range investigated. The momentum spread increased from O.1 to 1.3% over the same range of bunch charge. Comparison of these measured values with PARMELA simulations indicated that the program predicted the transverse emittance to within 70% and the momentum spread to within 50%. This level of agreement establishes the utility of PARMELA for the design of high-brightness electron guns in a new regime. Finally, by assuming a Maxwellian velocity distribution, we inferred the average transverse energy (kB T) of the electrons emitted by our GaAs wafer to be 0.20 ± 0.03 eV at a laser wavelength of 527nm.